Maybe that's it. The herbs season the fat. For this batch I used a sprinkle of Herbs du Provence and whole thyme sprigs. I made sure I got some of it in the bag. I'm not sure it makes more of a difference, but what I can say is that it is really really good. And when you pull the legs out of the bag and roast them, and the fat starts roasting the herbs, man it smells really really good.
I've been eating these one at at time with a salad and the day's bread, the other day with some risotto, and half a bottle of red wine. I forget how good it is when you're actually sitting down and eating it rather than picking it off the bottom of a pan when I used to serve it to people.